Thanks for the clarification! I did manage to get the website up on Online Designer by loading it into Dropbox (see other discussion thread). Interestingly, when I first loaded it into dropbox, it did not show up for selection in Online Designer, but at some point later it did become visible.

I guess my expectation in following the link from Web Designer was that the project I was working on in Web Designer would be loaded into Online Designer 'automagically'. Might be worth Xara considering a step where they ask if user wants the current project to be loaded?